Is it really neccessary to get rid of them? Some things go out of date, but certain principles are constant. Before I retired in 2009, I was an practical Engineer, In my work I used Machinery's Handbook, it was the same one that my dad used before me, it dated to 1934. The laws for picking, and using, an index plate on the back of a mill are still the same. There was also the little Zeus that every practical engineer has, with it's tables, etc. a formulae and drill sizes are valid today as they were twenty years ago, as are the principals of maths, the laws of pythagoras are still the laws of pythagaras, trig, still works the same way.
Try online auctions, junk and charity shops, boot sales, the local schools.